Changes in relative enzyme activities during the lignification in the almond endocarp

Abstract:【Objective】Analysis of the relationship between lignin content in almond endocarp and related enzyme activity in the process of development reveals endocarp lignification and its mechanism.【Methods】This study used thin shelled'Zhipi'and thick shelled'Changshitou'as the experimental materials. Lignin content, cellulose content, polyphenol content and related enzyme activities in almond endocarp were determined in different development periods and the correlations between various parameters were analyzed.【Results】The endocarp development in the thin shelled'Zhipi'and thick shelled'Changshitou'was consistent. Lignin content showed a trend of rising. From 25 to 40 d after full blossom, 'Zhipi'and'Changshitou'accumulated lignin slowly. The lignin content in'Zhipi'was higher than in'Changshitou'during this period. Rapid lignin accumulation started at different times. The rapid increase in lignin content in'Changshitou'started around 40 days after full blossom, and it started from 55 d after full blossom in'Zhipi'.'Changshitou'was thus 15 days earlier than'Zhipi'. At 55 d after full blossom, the lignin content in'Zhipi'and'Changshitou'rose rapidly, and in'Changshitou', lignin content was higher than in'Zhipi'. During the endocarp development, cellulose content tended to rise in both varieties. At first, the cellulose content in'Zhipi'was higher than in'Changshitou'.Starting around 35 d after full blossom, 'Changshitou'had a higher lignin content than'Zhipi'. The polyphenol content declined constantly. At first, it was higher in'Changshitou', but from 45 d after fullblossom, it declined faster in'Changshitou'than in 'Zhipi'. The change patterns of C4H, CAD and PAL activities were similar. They displayed a downward trend after an initial rise. The enzyme activities in'Zhipi'reached a peak at 55 d after full blossom, while those in'Changshitou'at 70 d after full blossom. From 25 to 70 d after full blossom, there were no significant differences between varieties. At 70 d after full blossom, the enzyme activities in'Changshitou'were higher than in'Zhipi'. The occurrence the enzyme activity peak was coincide with the rapid accumulation of lignin. The study also found that the enzyme activity dropped to certain level, but rapid accumulation of lignin continued. The endocarp of the thick shelled'Changshitou'had constantly significantly higher activities of POD and PPO than that of the thin shelled'Zhipi', indicating that thicker endocarp needs to have higher the POD and PPO activities. In the whole process of endocarp development'Changshitou'always had a higher PPO activity than'Zhipi'. In, 'Zhipi'endocarp, lignin content and activity of CAD were significantly correlated, and cellulose content and PPO activity had a significant positive correlation during the early stage. During late period of rapid endocarp growth, lignin content was significantly and negatively correlated to C4H and CAD activities, and cellulose content had significant negative correlations with PPO, PAL, C4H and CAD activities, while polyphenol content and POD activity had a significant positive correlation. In'Changshitou', during the early stage of endocarp development, a significant negative correlation was found between polyphenol and lignin contents; cellulose content was significantly positively correlated to PAL, C4H and CAD activities; and polyphenol content was significantly and negatively correlated to PAL and CAD activities. During the late period of rapid endocarp development, significant negative correlations were found between lignin content and PPO activity and between cellulose content and PPO activity.【Conclusion】Higher activities of PAL, CAD, C4H, PPO and POD as well abundant lignin precursors enables greater accumulation of lignin, which generates a thicker lignified endocarp as shown in the thick shelled'Changshitou'.